a) Field of the Invention
The invention is directed to a mode-synchronized solid-state laser comprising a laser medium inside of a laser cavity or laser resonator which is formed by a cavity mirror or resonator mirror and an output coupling mirror and which is folded by means of at least one concave folding mirror, with a saturable absorber inside the laser resonator and with at least one laser pump source whose pump beam pumps the laser medium, wherein the elongated resonator length L is a function of the pulse repetition frequency and the latter is determined by a distance between the resonator mirror and the output coupling mirror.
b) Description of the Related Art
Mode-synchronized solid-state lasers generally require long resonators. The resonator length L determines the repetition frequency v according to the known formula:       v    =          c              2        ·        n        ·        L              ,
where c is the speed of light and n is the index of refraction. Typical frequencies are 50 MHZ to 300 MHZ, resulting in resonator lengths of approximately 3 m to 0.5 m, for example. In order to reduce the constructional length and to achieve sufficient stability, these resonators must usually be folded. When folding, astigmatism results with oblique incident light on spherical surfaces, so that the quality of the laser beam is appreciably worsened.
Further, the quality of the laser beam is worsened by the thermal lens, as it is called. The effect of the thermal lens and the conventional compensation in laser resonators is described in W. Koechner, xe2x80x9cSolid-State Laser Engineeringxe2x80x9d, Springer, 1996. The compensation of the thermal lens requires the use of spherical optics or a corresponding dimensioning of the active medium.
A beam with TEM00 and M2xcx9c1 is required for mode locking. Typical resonators for mode-synchronized lasers are described in Keller, U., xe2x80x9cUltrafast All-Solid-State Laser Technologyxe2x80x9d, Appl. Physics B, 58, 347-363 (1994) or in Keller, U., T. Heng Chiu, xe2x80x9cResonant Passive Mode-Locked Nd:YLF Laserxe2x80x9d, IEEE J. of QE vol. 28, 1710-1720 (1992). For the resonators described therein, the passive saturable absorber is typically located in an intracavity focus with high intensity. This is achieved by means of short-focal-length focusing mirrors or by beam-bundling lenses. The active spot is kept small (Ø50-200 xcexcm). This is required in order to achieve increasingly shorter pulse widths in the femtosecond range.
In practice, it has also been shown that the homogeneity of commercially available saturable absorbers is poor and therefore only a relatively small portion of the surface of a saturable absorber component ever fulfills its function in an optimal manner. Accordingly, a small focus (under 200 xcexcm) is also advantageous on the saturable absorber; however, this limits the output power of the laser.
Due to the astigmatism of the folded resonator and the influence of the thermal lens in the laser medium, only a small beam cross section (e.g., 50 xcexcm) is possible in order to achieve an approximately homogeneous beam bundle in the laser resonator. Therefore, the passive mode synchronization with a saturable absorber based on semiconductors was formerly limited to relatively small average outputs ( less than 1 W). The laser destruction threshold of semiconductor materials of this kind is typically in the range of 1 MW/cm2 to 10 MW/cm2 (CW). Due to the saturable absorption and a remaining proportion of residual absorption, this value is again substantially reduced and must be taken into account in the design of the resonator. This led to various suggestions for reducing the output density and/or the output distribution in the saturable absorber.
DE 196 80 508 (Nighan) discloses diode-pumped solid-state lasers for higher average outputs in which the beam guidance in the laser resonator is adjusted in such a way that the beam bundle from the resonator mirror into the laser medium is neither confocal nor concentric (page 5, lines 20 ff: xe2x80x9cbetween confocal and concentricxe2x80x9d or xe2x80x9cconfocal-to-concentricxe2x80x9d or xe2x80x9calmost confocalxe2x80x9d (page 12, line 25)). In this case, a beam deformation occurs due to the folding in the confocal resonator as well as due to the influence of the thermal lens. This arrangement is not suitable for passive mode locking since the geometry of the resonator is essentially determined by the thermal lens.
U.S. Pat. No. 5,812,308 (Kafka) discloses a mode-synchronized laser with amplifier. The distinguishing aspect of this system consists in that the laser crystal is located directly in front of the folding mirror and is pumped by the latter. The optical axis of the resonator traverses the crystal twice in the zone excited by the pump optics. The angle between the two parts of the optical axis is relatively small due to the limited dimensions of the laser medium. However, with this small angle, the astigmatism is kept comparatively slight, but severe restrictions result for the design of resonator geometry. The average output is limited by the pump geometry. A passive saturable absorber based on semiconductors with a quantum well is indicated as mode locker and simultaneously serves as a resonator mirror. It is disadvantageous that this arrangement allows only relatively small folding angles because of the laser crystal geometry.
A similar arrangement, although with a plurality of crystals, is described in U.S. Pat. No. 5,237,584 (Shannon). In this case also, the beam quality is far removed from an intracavity beam bundle with TEM00 and M2xcx9c1 because of the plurality of folds via the laser crystals and the plurality of intracavity surfaces and is therefore not suitable for mode synchronization with high output.
WO 98/02945 (Nighan) discloses a Nd-Vanadate laser with Q-switching. A single-folded resonator is used for this solution. The folding is carried out via a plane mirror and the pump light is radiated along the optical axis of the resonator. In order to minimize astigmatism, the folding mirror is flat. This arrangement is not suitable for mode synchronization because there is no possibility in this very short resonator (18 cm) for the arrangement of the mode locker or for the corresponding adjustment of the pulse energy for saturation of the passive absorber.
WO 95/21479 (Keirstaed) shows how the ellipticity of the thermal lens can be reduced principally for the Nd:YVO4 crystal. This is effected by purposeful dimensioning of the heat sink and use of the anisotropy of this crystal. It is disadvantageous that the heat of the pump light which is not converted into laser radiation is preferably carried off only along two surfaces in the crystal. Accordingly, the effective cooling of the laser crystal is reduced, which leads to loss of effectiveness especially with.high laser outputs. The radial symmetry of the thermal lens is suitable for linear resonators in which there is no astigmatism. However, it has been determined that the effect of the pump light distribution on the beam quality is appreciably greater than that of the heat sink.
The primary object of the invention is to provide a mode-synchronized solid-state laser whose laser resonator is constructed in a particularly simple manner and which achieves the beam parameters TEM00 and M2=1 with the closest possible approximation more easily or with simpler steps and whose pulsed output radiation is as close as possible to being 100% stable. The invention has the further aim of providing a mode-synchronized solid-state laser and to provide its laser resonator in particular with a pulse repetition frequency between 50 MHZ and 300 MHZ and especially with a pulse duration between 0.1 and 100 picoseconds with higher amplitude stability, especially with an output power greater than 1 Watt and with an almost diffraction-limited laser beam.
The invention is directed to a mode-synchronized solid-state laser comprising a laser medium inside a laser resonator which is formed by a resonator mirror and an output coupling mirror and which is folded by means of at least one concave folding mirror, with a saturable absorber inside the laser resonator and with at least one laser pump source whose pump beam pumps the laser medium, wherein the elongated resonator length L is a function of the pulse repetition frequency and the latter is determined by a distance between the resonator mirror and the output coupling mirror.
The invention is characterized in that an elliptic beam profile inside of the laser resonator which is caused by an inclination of an optical axis of the at least one concave folding mirror relative to a folded optical axis of the laser resonator is eliminated by an elliptic thermal lens in the laser medium whose optical action generates an ellipticity of the beam profile which is offset by 90xc2x0 relative to the elliptic beam profile generated by the folding mirror, wherein, by means of the material characteristics, the crystallographic orientation and the geometric dimensioning of the laser medium and by a choice of intensity and intensity distribution of the pump beam in the laser medium, the degree of ellipticity of the thermal lens is adjusted in such a way that the most uniform possible energy density distribution occurs over the irradiated zone of the saturable absorber.
Such compensation of the astigmatism of the folding mirror by the astigmatism of the thermal lens is achieved in a particularly simple manner when the following geometric boundary conditions are adhered to. The radius of curvature R of the folding mirror is advisably in the range of 0.5-times to 2-times the mean focal length f of the thermal lens in order to minimize its influence on the beam path in the resonator irrespective of the absolute value of the focal length f. In particular, the radius of curvature R of the folding mirror is approximately equal to the value of the average focal length f of the thermal lens.
With this dimensioning, the thermal lens has the least influence on the beam path in the resonator, since the beam-deforming action of the folding mirror and thermal lens compensate one another. Further, it is advantageous when the concave folding mirror has a radius of curvature R between L/2 and 2*L, in particular, the radius of curvature R is approximately equal to the laser resonator length L. For this purpose, this folding mirror is arranged within the elongated resonator length L and forms a first resonator branch with length l1 and a second resonator branch with length l2 and the laser medium is arranged therein with the thermal lens.
At a pulse repetition frequency of 160 MHZ, for example, the resonator length L is about 93 cm, the radius R of the folding mirror is about 75 cm. The average focal length f of the thermal lens should be greater than 60 cm, for example, about 80 cm.
To compensate for beam deformation (astigmatism), it is further advantageous when the lengths are adapted to one another in determined orders of magnitude: Let l1+l2=L, where l2 is greater than l1. In particular, l2 is two-times to three-times greater than l1.
An energy density must be adjusted on the surface of the saturable absorber such that the latter is brought to the saturated state once within its charge carrier lifetime, i.e., a single pulse is generated. This is achieved in that, with a given radius R of the folding mirror and a given resonator length L, the distance between the folding mirror and the resonator mirror can be adjusted with the one change in length such that the size of the irradiated zone (spot) on the saturable absorber is adjustable in such a way, depending on the radiation output impinging on the latter, that an energy density is achieved in order to saturate the saturable absorber once within its charge carrier lifetime, wherein the elongated resonator length L is retained by a further change in length between the folding mirror and the output coupling mirror.
The irradiated zone on the saturable absorber is advantageously greater than 200 xcexcm in diameter. This ensures that the energy density is safely below a destruction threshold of the saturable absorber even at high laser output powers.
An advantageous arrangement of the invention is achieved when the folding mirror adjusts a fold angle xcex2 of the optical axis of the resonator between 5xc2x0 and 45xc2x0 in a fold plane. A spatially compact construction of the laser resonator is achieved in this way.
The saturable absorber is arranged at a distance l4 from the resonator mirror which is less than l1/10. The beam profile of the laser beam impinging on the resonator mirror is then almost identical to that impinging on the saturable absorber. Because of the separate arrangement, each structural component part can be independently produced and adjusted in the laser resonator.
It is particularly advantageous when the saturable absorber is arranged on the resonator mirror. However, this is only possible when the corresponding manufacturing techniques for a resonator mirror of this type have been successfully mastered and a component group of this kind delivers sufficiently good pulsed high-power laser radiation. This is achieved in that the saturable absorber comprises a single quantum layer (single quantum well) which is arranged on the resonator mirror.
Further, it is advantageous for the saturably absorbing action of the saturable absorber to provide at least one optical surface of the saturable absorber adjoining the free volume (air) with an anti-reflection coating.
The laser pump source is formed of one or more laser diodes which is/are optically connected with the laser medium via coupling optics. The pump optics can also be outfitted with astigmatic optics in order to influence the shape of the thermal lens in a corresponding manner.
An advantageous arrangement for instrument design consists in that a light-conducting fiber connection is provided between the laser diode(s) and the coupling optics in each instance.
The resonator mirror is a metal mirror or a dielectric mirror, in particular a Bragg reflector.
The surface normal of each end face of the laser medium advantageously extends at a small angle of between 0xc2x0 and 5xc2x0 relative to the optical axis of the laser resonator in order to prevent interfering reflections.
A particularly compact construction of the laser resonator is achieved in that this laser resonator is additionally folded once or twice by one or two input coupling mirrors, and the one or two laser pump source(s) pump(s) the laser medium at the ends with its (their) pump beam(s) via the input coupling mirror(s). The folding is carried out at an angle xcex3 which, in particular, is equal to xcex2. With one pump source at each end of the laser medium, the shape of the thermal lens can be adjusted very favorably. The pump light distribution in the laser medium is substantially homogeneous in the z-direction. The shaping of the thermal lens is not strict; its focal length can be adjusted so as to be comparatively great, which in turn enables a substantially higher laser output power.
The saturable absorber lies outside of the beam path defined by the end face of the laser medium and the input coupling mirror, i.e., the input coupling mirror is at a distance of less than l3/2 from the end face of the laser medium.
Another possibility consists in that the laser medium is pumped from the side, wherein the elliptic thermal lens and the inversion distribution are adjustable by means of a distributed arrangement of the radiation of a plurality of laser pump sources along the circumferential surfaces of the laser medium.
Further, for fine adjustment of the intensity distribution on the saturable absorber, an inclination of an optical axis of the pump beam(s) is provided relative to an optical axis of the laser resonator in departure from a 0-degree position (end-pumped) or 90-degree position (side-pumped) at an angle xcex1 in a range greater than 0xc2x0 to less than 5xc2x0 within the y-z plane and/or the x-z plane, wherein, in particular, the ellipticity of the thermal lens can be finely adjusted with this inclination. As a consequence of this fine adjustment, it is possible to influence the spot on the saturable absorber in such a way that a faultless mode synchronization is effected. The inversion distribution in the laser crystal is adjustable by the inclination of the optical axes of the pump light, so that a matching homogeneous active spot occurs on the passive mode locker, allowing output powers of greater than 4 W.
Accordingly, larger homogeneous active beam diameters are required on the mode locker for increasing the average outputs of mode-synchronized diode-pumped solid-state lasers operating with passive mode synchronization and can be realized by the features of the invention. In order to achieve this, the laser resonator must be adapted and optimized, as was described above, in that the beam inhomogeneities resulting from the folding of the beam path, the effect of the thermal lens and the inversion distribution in the laser medium due to the pump light distribution are utilized in an advantageous manner in order to generate a pulsed laser beam with high output and constant beam parameters. However, the saturable absorber itself must likewise be sufficiently homogeneous.
A mode diaphragm which is arranged in the beam path between an end face of the laser medium and the saturable absorber generates the desired mode distribution, especially a TEM00 mode of the laser beam.
Taking into account the above-mentioned conditions, a construction is advantageous in which only the output coupling mirror is in the first resonator branch and the laser medium with the thermal lens, the saturable absorber and the resonator mirror are arranged in the second resonator branch successively proceeding from the folding mirror. In this case, a distance l3 from the laser medium to the resonator mirror is less than l1 and, in particular, l3 is one-half to one-third as large as l1. The spot size on the saturable absorber is adjusted by the change in distance xcex94l2. Also, the output coupling mirror is adjusted by xcex94l1 to maintain the resonator length L. Alternatively, the folding mirror can also be displaced by xcex94l and the output coupling mirror or the resonator mirror is adjusted.
In another variant, the saturable absorber and the resonator mirror are arranged in the first resonator branch successively proceeding from the folding mirror and the laser medium with the thermal lens and the output coupling mirror are arranged in the second resonator branch proceeding from the folding mirror. In this case also, the size of the spot on the saturable absorber can be adjusted by displacing the mirrors, wherein different scale ratios prevail in this case due to the changed length ratios.
It is expressly noted that the resonator geometry described above is presented by way of example. The principles of the invention are also applicable in particular with laser resonators which have a plurality of spherical folding mirrors in a laser resonator and/or which contain a plurality of laser media and/or in which the individual components have different distance ratios relative to one another.
The invention will be described in the following with reference to the FIGURES.